linux-kernel - Re: [PATCH] sched_rt_rq

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Date:	Wed, 27 Aug 2008 09:05:38 -0400
From:	John Blackwood <john.blackwood@...r.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] sched_rt_rq_enqueue() resched idle

 > On Tue, 2008-08-26 at 15:09 -0400, John Blackwood wrote:
 > > > Hi Peter,
 > > >
 > > > When sysctl_sched_rt_runtime is set to something other than -1 
and the
 > > > CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get 
into a state
 > > > where we see one or more CPUs idling forvever even though there are
 > > > real-time
 > > > tasks in their rt runqueue that are able to run (no longer 
throttled).
 > > >
 > > > The sequence is:
 > > >
 > > > - A real-time task is running when the timer sets the rt runqueue
 > > >     to throttled, and the rt task is resched_task()ed and switched
 > > >     out, and idle is switched in since there are no non-rt tasks to
 > > >     run on that cpu.
 > > >
 > > > - Eventually the do_sched_rt_period_timer() runs and un-throttles
 > > >     the rt runqueue, but we just exit the timer interrupt and go back
 > > >     to executing the idle task in the idle loop forever.
 > > >
 > > > If we change the sched_rt_rq_enqueue() routine to use some of the 
code
 > > > from the CONFIG_RT_GROUP_SCHED enabled version of this same 
routine and
 > > > resched_task() the currently executing task (idle in our case) if 
it is
 > > > a lower priority task than the higher rt task in the now un-throttled
 > > > runqueue, the problem is no longer observed.
 >
 > Very good spotting, Thanks!

You're welcome.

 > However I think the patch isn't quite good, as highest_prio is only
 > available on SMP || RT_GROUP_SCHED.
 >
 > Furthermore, on !RT_GROUP_SCHED any RT task will be higher than current,
 > so we can do the below, do you agree?

Yes, I see what you are saying.
The patch version below looks good.
I re-tested with it and it works fine.

Thanks!

 > diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
 > index 94daace..f672aee 100644
 > --- a/kernel/sched_rt.c
 > +++ b/kernel/sched_rt.c
 > @@ -199,6 +199,8 @@ static inline struct rt_rq *group_rt_rq(struct 
sched_rt_entity *rt_se)
 >
 >  static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
 >  {
 > +       if (rt_rq->rt_nr_running)
 > +               resched_task(rq_of_rt_rq(rt_rq)->curr);
 >  }
 >
 >  static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
 >
 >
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